Sound recordings are made by converting sound vibrations into surface deformities in a soft surface, such as a layer of lacquer on a metal disc. These deformities are then replicated in metal, as by metal plating of the lacquered surface, to form a stamping master from which a plurality of plastic reproductions can be made.
A capacitive method of recording and playback of video as well as sound information has recently been disclosed by Clemens in U.S. Pat. No. 3,824,194. According to the method described in the patent, an aluminum blank is coated with lacquer and a spiral groove cut into the lacquer. A silver layer is then chemically deposited on the lacquer and replicated to form a recording master. Audio, video and color information is recorded in the groove using a scanning electron microscope which modulates the groove with the information to be recorded. When the electron beam resist is developed, the information is in the form of a surface relief pattern in the groove.
A metal replica is then made from the developed recording master. One known method is to apply a layer of silverplate onto the soft surface followed by a low stress sulfamate nickel layer as a backing layer, or, an electroless nickel layer can be applied followed by a nickel plate layer. Another method is described by Nosker and Fox in U.S. Pat. No. 3,904,488. The original recording and the metal replicas are separated and the metal replica is then used to compression mold or stamp a plurality of plastic disc replicas.
Several changes in the basic recording system have since been made. Presently, the information is initially recorded in a metal master by electromechanical means, as is described in patents to Halter, inter alia, U.S. Pat. Nos. 4,044,379 and 4,060,831. The initial metal recording can be readily duplicated by electroplating, as with nickel, to form a fairly thick (17.5 mils or 444.5 microns) master. A mold is then made from the master by electrolessly plating a thin layer (1/2 mil or 12.7 microns) of nickel on the surface followed by electroplating a nickel backing layer. This produces a positive replica of the intial recording. A metal stamper is then made from the mold by electrolessly plating a 1/2 mil (12.7 microns) thick nickel layer followed by electroplating a 7 mil (177.8 microns) thick nickel backing layer. This negative image stamper is then used for compression molding to produce the plastic replicas.
The original video disc disclosed by Clemens comprised a plastic disc coated with a thin coating of metal and a thin dielectric layer. More recently a video disc has been developed which contains conductive carbon particles dispersed in the molding composition. The carbon particles are submicron sized, irregularly shaped particles present in the molding composition in fairly high amounts, e.g., from about 12-35 percent by weight of the molding composition. This comparatively high loading of abrasive particles has caused wearing of the nickel molding stampers in the form of a plurality of tiny permanent scratches. Other abrasive particles may be present as impurities in the vinyl resin, additives and the like. As the conductive molding composition is compression molded, the material is squeezed from the center outwardly in the mold and the abrasive carbon and any other hard particles in the molding composition can scratch the surface of the stamper as the material flows across the stamper face during molding. As succeeding discs are molded, the stamper face becomes increasingly scratched, which scratches are replicated in succeeding discs, resulting in unwanted dropouts and other electronic defects during playback of the disc. Although hundreds and even thousands of discs can be made from a single stamper using conventional PVC molding compositions in the record industry, using the carbon loaded composition the stampers become unuseable after only 100 or 200 replicas have been molded. Commercial operation requires that 800 to 1200 disc replicas be able to be made from a single stamper. Since only a limited number of metal stampers can be made from a single original recording, it would of course be highly desirable to increase the life of the metal stampers and increase the number of high quality disc replicas that can be made from each stamper.